Method of making partial-discharge-resistant insulated wire

ABSTRACT

A partial-discharge-resistant insulating varnish has a polyamide-imide enamel varnish and an organo-silica sol that are dispersed in a solvent. The solvent has 50 to 100% by weight of γ-butyrolactone. An insulated wire has a conductor, and a partial-discharge-resistant insulation coating film formed on the surface of the conductor. The partial-discharge-resistant insulation coating film is made of the partial-discharge-resistant insulating varnish.

The present application is a divisional of U.S. application Ser. No.11/303,909, filed Dec. 19, 2005, which claims benefit of priority fromthe prior Japanese patent application No. 2005-126810, the entirecontents of all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a partial-discharge-resistant insulatingvarnish, an insulated wire, and a method of making the same. Inparticular, this invention relates to: a partial-discharge-resistantinsulating varnish that comprises a mixture of γ-butyrolactone as asolvent component, polyamide-imide enamel varnish and organo-silica sol;an insulated wire that a film of the partial-discharge-resistantinsulation varnish is formed on a conductor; and a method of making thesame.

2. Description of the Related Art

The partial discharge is generated such that, when a minute gap existsin an insulation for a wire or cable or between wires, electric fieldconcentrates on that part to cause a weak discharge. Due to the partialdischarge generated, the insulation deteriorates. Further, due to theprogress of the deterioration, a breakdown will occur.

Especially, in windings used for a motor or transformer, for example, inenameled wires that enamel varnish is coated on a conductor and thenbaked to make a coating film thereon, the partial discharge can begenerated mainly between the wires (between the coating films) orbetween the coating film and the core. Thus, erosion of the coating filmmay progress mainly due to cutting of molecular chain in the resincoating film or heat generation caused by collision of chargedparticles. As a result, the breakdown may occur.

In recent years, in an inverter-fed motor system used for energy savingor adjustable speed, many cases have been reported in which invertersurge (steep overvoltage) is generated to cause the motor breakdown. Itis found that the motor breakdown is caused by the partial discharge dueto the overvoltage of the inverter surge.

In order to prevent the partial discharge erosion, an enameled wire isknown which has an insulation made of a enamel varnish that inorganicinsulating particles such as silica and titania are dispersed in aheat-resistant resin solution with an organic solvent. Such an inorganicinsulating particle can provide the enameled wire with the partialdischarge resistance, and can further contribute to enhancement inthermal conductivity, reduction in thermal expansion and enhancement instrength.

Known methods of dispersing a silica fine particle as the inorganicinsulating particle in a resin solution are such as a method of addingand dispersing a silica fine particles powder into the resin solution,and a method of mixing the resin solution and a silica sol (for example,JP-A-2001-307557). As compared to the method of adding the silicaparticles powder thereinto, the method of using the silica sol canfacilitate the mixing and can offer the varnish that the silica is welldispersed. However, in this case, the silica sol needs a highcompatibility with the resin solution.

When a polyimide-imide insulating material is used as the heat-resistantpolymer, a solvent to this can be N-methyl-2-pyrrolidone (NMP),N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC),dimethylimidazolidinone (DMI) etc. In general, a solvent is used whichcontain mainly NMP and is diluted with DMF, aromatic alkylbenzene etc.

However, conventionally, when such a polyamide-imide enamel varnish withthe solvent containing NMP as the main component is used to disperse thesilica fine particles thereinto, the silica fine particles areaggregated not to allow the sufficient dispersion. There is acorrelation between the partial discharge resistance of the wire coatingfilm and the surface area of silica particles in the wire coating film.If the coating film is formed by using a silica-dispersed enamel varnishwith insufficient dispersion, i.e., with many aggregates, the partialdischarge resistance of the coating film must be reduced. Therefore, thesilica fine particles need to be uniformly dispersed without theaggregates in the coating film.

On the other hand, when the organo-silica sol is used as a silicasource, it is prepared by dispersing silica fine particles into anorganic solvent such as DMAC, DMF, alcohol and ketone. However, such anorgano-silica-sol has a low compatibility with the polyamide-imide resinbeing dissolved in the NMP, so that the aggregates will be likelygenerated. Further, even if a uniform dispersion can be obtained underlimited conditions, there will be generated problems in long-termkeeping quality, stability, and reproducibility.

SUMMARY OF THE INVENTION

It is an object of the invention to provide apartial-discharge-resistant insulating varnish that silica fineparticles can be uniformly dispersed preventing the aggregation thereofso as to enhance the partial discharge resistance.

It is another object of the invention to provide an insulated wire thata coating film is formed on a conductor by using thepartial-discharge-resistant insulating varnish.

It is another object of the invention to provide methods of making thepartial-discharge-resistant insulating varnish and the insulated wire.

(1) According to one aspect of the invention, apartial-discharge-resistant insulating varnish comprises:

a polyamide-imide enamel varnish and an organo-silica sol that aredispersed in a solvent,

wherein the solvent comprises 50 to 100% by weight of γ-butyrolactone.

In the above invention, the following modifications or changes may bemade.

(i) a silica component of the organo-silica sol is 1 to 100 phr (partsper hundred parts of resin) by weight to a resin component of thepolyamide-imide enamel varnish.

(ii) the organo-silica sol has an average particle size of 100 nm orless.

(2) According to another aspect of the invention, an insulated wirecomprises:

a conductor; and

a partial-discharge-resistant insulation coating formed on the surfaceof the conductor,

wherein the partial-discharge-resistant insulation coating film is madeof the partial-discharge-resistant insulating varnish as describedabove.

In the above invention, the following modifications or changes may bemade.

(iii) The insulated wire further comprises an organic insulation coatingfilm formed on the surface of the conductor,

wherein the partial-discharge-resistant insulation coating film isformed on the surface of the organic insulation coating film.

(iv) The insulated wire further comprises an other organic insulationcoating film formed on the surface of the partial-discharge-resistantinsulation coating film.

(3) According to another aspect of the invention, a method of making apartial-discharge-resistant insulating varnish comprises:

mixing a polyamide-imide enamel varnish with an organo-silica sol,

wherein the polyamide-imide enamel varnish comprises γ-butyrolactone asa main solvent,

the organo-silica sol comprises γ-butyrolactone as a main dispersionsolvent, and

the partial-discharge-resistant insulating varnish comprises 50 to 100%by weight of γ-butyrolactone to a total amount of a solvent thereof.

In the above invention, the following modifications or changes may bemade.

(v) the polyamide-imide enamel varnish comprises 60 to 100% by weight ofγ-butyrolactone to a total amount of a solvent thereof.

(vi) the organo-silica sol comprises 80 to 100% by weight ofγ-butyrolactone to a total amount of a dispersion solvent thereof.

(4) According to another aspect of the invention, a method of making aninsulated wire comprises:

preparing a partial-discharge-resistant insulating varnish by mixing apolyamide-imide enamel varnish with an organo-silica sol; and

coating the partial-discharge-resistant insulating varnish on thesurface of a conductor and then baking the varnish to form a coatingfilm on the conductor,

wherein the polyamide-imide enamel varnish comprises γ-butyrolactone asa main solvent,

the organo-silica sol comprises γ-butyrolactone as a main dispersionsolvent, and

the partial-discharge-resistant insulating varnish comprises 50 to 100%by weight of γ-butyrolactone to a total amount of a solvent thereof.

In the above invention, the following modifications or changes may bemade.

(vii) The method further comprises forming an organic insulation coatingfilm on the surface of the conductor, wherein thepartial-discharge-resistant insulation coating film is formed on thesurface of the organic insulation coating film.

Advantages of the Invention

The partial-discharge-resistant insulating varnish with enhanced partialdischarge resistance can be obtained since the organo-silica sol isuniformly dispersed preventing the aggregation thereof.

The insulated wire can be less likely to be subjected to the partialdischarge erosion since the conductor is coated by thepartial-discharge-resistant insulating varnish with the organo-silicasol uniformly dispersed such that the insulation coating film can beformed with the silica uniformly dispersed. As a result, the insulatedwire can be applied to various inverter-fed systems to significantlyelongate the lifetime of electric appliances therewith.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments according to the invention will be explainedbelow referring to the drawings, wherein:

FIG. 1 is a cross sectional view showing an insulated wire in apreferred embodiment according to the invention;

FIG. 2 is a cross sectional view showing an insulated wire in anotherpreferred embodiment according to the invention; and

FIG. 3 is a cross sectional view showing an insulated wire in anotherpreferred embodiment according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Organo-Silica Sol

The organo-silica sol used in the invention has preferably an averageparticle diameter (in the BET method) of 100 nm or less, more preferablyof 30 nm or less so as to effectively provide the coating film with thepartial discharge resistance. In case of 30 nm or less, theorgano-silica sol itself has an enhanced transparency.

When γ-butyrolactone is as a main dispersion solvent for organo-silicasol, the compatibility of the sol with the resin solution can beenhanced to prevent the aggregation or increase in viscosity when beingmixed. The dispersion solvent may contain, mixing with γ-butyrolactone,a polar solvent such as NMP and DMF, aromatic hydrocarbon or loweralcohol in order to enhance the stability. However, as the ratio of themixed solvents increases, the compatibility with the resin solution willlower. Thus, the ratio of γ-butyrolactone is desirably 80% or more.

The organo-silica sol can be prepared by conducting the solventreplacement to a silica sol obtained by the hydrolysis of alkoxysilaneor by a silica sol obtained by the ion exchange process of waterglass(sodium silicate). However, the organo-silica sol may be prepared by theother known method other than the above methods.

The suitable amount of moisture in the organo-silica sol may be varieddepending on the composition of the mixed solvents for the dispersion.However, in general, when the amount is too much, the stability of thesol or the compatibility with the enamel varnish will lower. Therefore,the amount of moisture in the organo-silica sol is preferably 1.0% orless.

Since the organo-silica sol dispersed in the solvent with theabovementioned composition is excellent in dispersion property, theorgano-silica sol can be obtained with a high silica concentration of20% or more.

Polyamide-Imide Enamel Varnish

The polyamide-imide enamel varnish can be prepared by the synthesisreaction that 4,4′-diphenylmethane diisocyanate (MDI) and trimelliticanhydride (TMA) are reacted at equimol quantities in a solvent with NMPas a main component, which is most typically used from the aspect ofproperty, cost or availability of materials. However, if the heatresistance of 200° C. or more can be kept in the polyamide-imideenameled wire, the raw material structure of aromatic isocyanates,aromatic carboxylic acids, and acid anhydrides is not limitedspecifically. Thus, it can be also prepared by known synthesis methodsto react aromatic diamines such as 4,4′-diaminodiphenylmethane (DAM)with acid chlorides such as trimellitic acid chloride (TMAC).

The solvent for the polyamide-imide enamel varnish can be alsoγ-butyrolactone as a main component such that the compatibility of thesol with the resin solution can be enhanced to prevent the aggregationor increase in viscosity when being mixed. The solvent may contain,mixing with γ-butyrolactone, a polar solvent such as NMP and DMF,aromatic hydrocarbon or lower alcohol in order to enhance the stability.However, as the ratio of the mixed solvents increases, the compatibilitywith the resin solution will lower. Thus, the ratio of γ-butyrolactoneis desirably 60% or more.

In order to prepare a polyamide-imide resin solution which containsγ-butyrolactone as a main solvent for polyamide-imide, any of knownmethods may be used such as: a method that polyamide-imide resinsynthesized in a solvent with NMP as a main component is precipitatedwith ethanol to collect only the resin fraction, then it is re-dissolvedin γ-butyrolactone; a method that the resin is directly synthesized in asolvent with γ-butyrolactone as a main component; and a method that thepolyamide-imide enamel varnish synthesized in a low-boiling pointsolvent such as DMF is solvent-replaced by γ-butyrolactone indistillation. However, in a solvent of 100% γ-butyrolactone, thepolyamide-imide is not synthesized with good reactivity. Therefore, acatalyst such as amines and imidazolines can be used therein. However,since γ-butyrolactone has a solubility of resin less than NMP etc., acompound with a biphenyl structure cannot be used therein.

Mixing of Organo-Silica Sol and Polyamide-Imide Resin Solution

Then, the organo-silica sol with γ-butyrolactone as the main dispersionsolvent component is mixed with the polyamide-imide resin solution withγ-butyrolactone as the main solvent component. The solvent for theresultant partial-discharge-resistant enamel varnish can contain, mixingwith γ-butyrolactone, a polar solvent such as NMP and DMF, aromatichydrocarbon or lower alcohol in order to enhance the stability. However,as the ratio of the mixed solvents increases, the dispersion property ofthe silica particles in the enamel varnish will lower. Thus, the ratioof γ-butyrolactone is desirably 50% or more to the total amount of thesolvents.

Partial-Discharge-Resistant Insulating Varnish

In general, a resin material well dissolved in a solvent hastransparency even when it is colored. Also, insulating varnishes forenameled wires have generally transparency when they have no dispersedphase. The reason why the transparency is lost by the dispersionparticles is that visible light cannot be transmitted since thedispersion particle has a large size. Therefore, it can be easilydetermined by the transparency of enamel varnish whether fine particlesare uniformly dispersed or not. Similarly, it can be easily determinedby the transparency of a coating film whether the silica fine particlesare uniformly dispersed in the partial-discharge-resistant coating filmcoated on a conductor. Namely, when the predetermined amount of silicais dispersed, the effectiveness of the partial discharge resistanceproperty can be easily determined by the transparency of the coatingfilm.

In the embodiments of the invention, the polyamide-imide enamel varnishwith γ-butyrolactone as the main solvent is used instead of theconventional polyamide-imide enamel varnish with NMP as the mainsolvent, and the solvent is the same as the dispersion solvent for thesilica sol. Therefore, the compatibility can be enhanced such that theaggregation between silica particles, the precipitation of resin and theaggregation between silica and resin can be prevented when being mixed.Thus, a uniform varnish solution with transparency can be obtained.Also, when it is formed into coating film, the fine insulating coatingfilm with good smoothness can be obtained.

EXAMPLES

FIG. 1 is a cross sectional view showing an insulated wire in apreferred embodiment according to the invention.

The insulated wire is structured such that a partial-discharge-resistantinsulation coating film 2 is formed on a conductor 1. It is manufacturedby coating the abovementioned partial-discharge-resistant insulatingvarnish around the conductor 1 and then baking it.

FIG. 2 is a cross sectional view showing an insulated wire in anotherpreferred embodiment according to the invention.

This insulated wire is structured such that an organic insulationcoating film 3 is further formed around the partial-discharge-resistantinsulation coating film 2 as shown in FIG. 1 in order to enhance themechanical property (sliding property, scratch-resistant property etc.).

FIG. 3 is a cross sectional view showing an insulated wire in anotherpreferred embodiment according to the invention.

This insulated wire is structured such that an organic insulationcoating film 4 is formed on the conductor 1, thepartial-discharge-resistant insulation coating film 2 is formed on theorganic insulation coating film 4, and the organic insulation coatingfilm 3 is further formed around the partial-discharge-resistantinsulation coating film 2.

Method of Making an Enameled Wire

Examples 1-5 and Comparative examples 1-5 as described below aremanufactured as follows.

First, the polyamide-imide enamel varnish is prepared such that 300parts by weight of the solvent component is to 100 parts by weight ofthe polyamide-imide resin. The organo-silica sol is prepared such that300 parts by weight of the dispersion solvent component is to 100 partsby weight of the silica particles with an average particle diameter of12 nm.

Then, the organo-silica sol is added to the polyamide-imide enamelvarnish to have the partial-discharge-resistant insulating varnish. Inthis process, a preparation that 30 parts by weight of the silica isadded to 100 parts by weight of the resin portion in the polyamide-imideenamel varnish is agitated to have the partial-discharge-resistantinsulating varnish.

The resultant partial-discharge-resistant insulating varnish is coatedon a copper conductor with a diameter of 0.8 mm, and then baked to havean enameled wire with a coating film thickness of 30 μm. The enameledwire is evaluated in dimensions, appearance, and V-t characteristic.

Meanwhile, the V-t characteristic is a characteristic to indicate therelationship between a breakdown voltage and a breakdown time. 1 kVvoltage with sine waves of 10 kHz is applied to between twisted pairenameled wires, and a time up to the breakdown is measured.

Example 1

The polyamide-imide enamel varnish that 100% of the solvent component isγ-butyrolactone is mixed with the organo-silica sol that 100% of thedispersion solvent component is γ-butyrolactone to have thepartial-discharge-resistant insulating varnish. The amount ofγ-butyrolactone to the total amount of solvents is 100 wt %.

Example 2

The polyamide-imide enamel varnish with a mixed solvent that 80% of thesolvent component is γ-butyrolactone and 20% thereof is cyclohexanone ismixed with the organo-silica sol that 100% of the dispersion solventcomponent is γ-butyrolactone to have the partial-discharge-resistantinsulating varnish. The amount of γ-butyrolactone to the total amount ofsolvents is 84.6 wt %.

Example 3

The polyamide-imide enamel varnish with a mixed solvent that 85% of thesolvent component is γ-butyrolactone and 15% thereof is NMP is mixedwith the organo-silica sol that 100% of the dispersion solvent componentis γ-butyrolactone to have the partial-discharge-resistant insulatingvarnish. The amount of γ-butyrolactone to the total amount of solventsis 89.7 wt %.

Example 4

The polyamide-imide enamel varnish that 100% of the solvent component isγ-butyrolactone is mixed with the organo-silica sol that 40% of thedispersion solvent component is benzyl alcohol and 60% thereof issolvent naphtha to have the partial-discharge-resistant insulatingvarnish. The amount of γ-butyrolactone to the total amount of solventsis 76.9 wt %.

Example 5

The polyamide-imide enamel varnish that 67% of the solvent component isγ-butyrolactone, 10% thereof is DMF and 23% thereof is cyclohexanone ismixed with the organo-silica sol that 40% of the dispersion solventcomponent is benzyl alcohol and 60% thereof is solvent naphtha to havethe partial-discharge-resistant insulating varnish. The amount ofγ-butyrolactone to the total amount of solvents is 51.3 wt %.

Comparative Example 1

The polyamide-imide enamel varnish that 80% of the solvent component isNMP and 20% thereof is DMF is mixed with the organo-silica sol that 100%of the dispersion solvent component is DMF to have thepartial-discharge-resistant insulating varnish. The amount ofγ-butyrolactone to the total amount of solvents is 0 wt %.

Comparative Example 2

The polyamide-imide enamel varnish that 100% of the solvent component isNMP is mixed with the organo-silica sol that 100% of the dispersionsolvent component is DMAC to have the partial-discharge-resistantinsulating varnish. The amount of γ-butyrolactone to the total amount ofsolvents is 0 wt %.

Comparative Example 3

The polyimide-imide enamel varnish that 50% of the solvent component isγ-butyrolactone and 50% thereof is NMP is mixed with the organo-silicasol that 100% of the dispersion solvent component is DMF to have thepartial-discharge-resistant insulating varnish. The amount ofγ-butyrolactone to the total amount of solvents is 38.5 wt %.

Comparative Example 4

The polyamide-imide enamel varnish that 80% of the solvent component isNMP and 20% thereof is DMF is mixed with the organo-silica sol that 100%of the dispersion solvent component is γ-butyrolactone to have thepartial-discharge-resistant insulating varnish. The amount ofγ-butyrolactone to the total amount of solvents is 23.1 wt %.

Comparative Example 5

The polyamide-imide enamel varnish is obtained that 80% of the solventcomponent is NMP and 20% thereof is DMF. The amount of γ-butyrolactoneto the total amount of solvents is 0 wt %.

Table 1 shows the properties of the varnishes in Examples 1-5 andComparative examples 1-5, and the properties (dimensions, appearance,and V-t characteristic) of the enameled wires manufactured by using thevarnishes.

TABLE 1 Ex 1 Ex 2 Ex 3 Ex 4 Ex 5 Composition of polyamide-imide resin100 100 100 100 100 enamel varnish composition γ- 300 240 260 300 200 ofsolvent butyrolactone NMP 40 DMF 30 cyclohexanone 60 70 Composition ofsilica 30 30 30 30 30 organo-silica sol composition γ- 36 36 ofdispersion butyrolactone solvent benzyl alcohol 36 36 solvent naphthaDMF DMAC Amout of γ-butyrolactone to total amout of 100 84.6 89.7 76.951.3 solvents [wt %] Properties of partial- appearance transparenttransparent transparent transparent transparent discharge-resistantinsulating stability good good good good good varnish Properties ofdimensions conductor 0.800 0.800 0.800 0.800 0.800 enameled wire [mm]diameter coating film 0.030 0.031 0.030 0.030 0.030 thickness finishing0.860 0.861 0.860 0.860 0.860 outside diameter appearance transparenttransparent transparent transparent transparent V-t without 80.3 78.279.4 76.8 77.0 characteristic elongation [h] with 20% 44.6 45.3 45.842.8 46.7 10 kHz-1.0 kV elongation Com EX 1 Com EX 2 Com EX 3 Com EX 4Com EX 5 Composition of polyamide-imide resin 100 100 100 100 100 enamelvarnish composition γ- 150 of solvent butyrolactone NMP 240 300 150 240240 DMF 60 60 60 cyclohexane Composition of silica 30 30 30 30 —organo-silica sol composition γ- of dispersion butyrolactone solventbenzyl alcohol solvent naphtha DMF 90 90 DMAC 90 Amout ofγ-butyrolactone to total amout of 0 0 38.5 23.1 0 solvents [wt %]Properties of partial- appearance aggregated/ aggregated/ aggregated/aggregated/ transparent discharge-resistant insulating clouded cloudedclouded clouded varnish stability precipitated precipitated precipitatedprecipitated good Properties of dimensions conductor 0.800 0.800 0.8000.800 0.800 enameled wire [mm] diameter coating film 0.031 0.030 0.0300.030 0.030 thickness finishing 0.861 0.860 0.860 0.860 0.860 outsidediameter appearance whitened whitened whitened whitened transparent V-twithout 5.0 4.9 5.0 5.2 1.2 characteristic elongation [h] with 20% 1.41.3 1.2 1.1 0.9 10 kHz-1.0 kV elongation Ex 1-5: Examples 1-5, Com Ex1-5: Comparative examples 1-5

From the results of Table 1, it is found that thepartial-discharge-resistant insulating varnish in Examples 1-5, whichhas 50 wt % or more of γ-butyrolactone to the total amount of solvents,is transparent and good in stability. In contrast, it is found that thepartial-discharge-resistant insulating varnish in Comparative examples1-4, which has less than 50 wt % of γ-butyrolactone to the total amountof solvents, is aggregated and clouded, and not good in stability withprecipitation. Further, it is found that the enameled wire in Examples1-5 is transparent in appearance and excellent in V-t characteristic, ascompared to that in Comparative examples 1-5.

Furthermore, it is found that the partial-discharge-resistant insulatingvarnish in Examples 1-5 with the enamel varnish composition thatγ-butyrolactone is 60% or more of the solvent component is transparentin appearance and excellent in stability. Also, it is found that theenameled wire using the varnish is transparent in appearance andexcellent in V-t characteristic.

Although the invention has been described with respect to the specificembodiments for complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A method of making a partial-discharge-resistantinsulating varnish, comprising: mixing a polyamide-imide enamel varnishwith an organo-silica sol, wherein the polyamide-imide enamel varnishcomprises a polyamide-imide resin and a mixed solvent used for asynthesis reaction of the polyamide-imide enamel varnish, the mixedsolvent comprising 67 to 87% by weight of γ-butyrolactone and a balanceconsisting of 1) at least one of N-methyl-2-pyrrolidone andN,N-dimethylformamide, or 2) cyclohexanone and N,N-dimethylformamide,wherein the organo-silica sol comprises silica and a dispersion solventcomprising γ-butyrolactone, and wherein a total amount ofγ-butyrolactone comprised in the mixed solvent and the dispersionsolvent is 50% or more and less than 100% by weight relative to a totalamount of the mixed solvent and the dispersion solvent.
 2. The methodaccording to claim 1, wherein: the organo-silica sol comprises 80 to100% by weight of γ-butyrolactone to a total amount of the dispersionsolvent.
 3. The method according to claim 1, wherein: a total amount ofγ-butyrolactone comprised in the mixed solvent and the dispersionsolvent is 50 to 89.7% by weight relative to a total amount of the mixedsolvent and the dispersion solvent.
 4. A method of making an insulatedwire, comprising: preparing a partial-discharge-resistant insulatingvarnish by mixing a polyamide-imide enamel varnish with an organo-silicasol; and coating the partial-discharge-resistant insulating varnish onthe surface of a conductor and then baking the varnish to form a coatingfilm on the conductor, wherein the polyamide-imide enamel varnishcomprises a polyamide-imide resin and a mixed solvent used for asynthesis reaction of the polyamide-imide enamel varnish, the mixedsolvent comprising 67 to 87% by weight of γ-butyrolactone and a balanceconsisting of 1) at least one of N-methyl-2-pyrrolidone andN,N-dimethylformamide, or 2) cyclohexanone and N,N-dimethylformamide,wherein the organo-silica sol comprises silica and a dispersion solventcomprising γ-butyrolactone, and wherein a total amount ofγ-butyrolactone comprised in the mixed solvent and the dispersionsolvent is 50% or more and less than 100% by weight relative to a totalamount of the mixed solvent and the dispersion solvent.
 5. The methodaccording to claim 4, further comprising: forming an organic insulationcoating film on the surface of the conductor, wherein thepartial-discharge-resistant insulation coating film is formed on thesurface of the organic insulation coating film.
 6. The method accordingto claim 4, wherein: a total amount of γ-butyrolactone comprised in themixed solvent and the dispersion solvent is 50 to 89.7% by weightrelative to a total amount of the mixed solvent and the dispersionsolvent.